x86/tdx: Wrap exit reason with hcall_func()

TDX reuses VMEXIT "reasons" in its guest->host hypercall ABI.  This is
confusing because there might not be a VMEXIT involved at *all*.
These instances are supposed to document situation and reduce confusion
by wrapping VMEXIT reasons with hcall_func().

The decompression code does not follow this convention.

Unify the TDX decompression code with the other TDX use of VMEXIT reasons.
No functional changes.

Signed-off-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/all/20230505120332.1429957-1-nik.borisov%40suse.com